1. Field of the Invention:
The present invention relates to an oxygen sensor element to determine oxygen concentration in exhaust gases from vehicle engines with improved sensor performance particularly in unbalanced exhaust gases where the oxygen content is excess or less.
2. Brief Description of the Prior Art:
In a system purifying exhaust gas from vehicle engines, it is of significance to determine the oxygen concentration of exhaust gases for adjusting the proper composition of the exhaust gases leading into a catalyst converter within a given range. In particular, where there is used a "three way system" which can simultaneously remove non-combusted hydrocarbons (CH), carbon monoxide (CO) and nitrogen oxides (NOx), it is necessary to adjust the exhaust gas composition within a very narrow range around the theoretical air-to-fuel ratio in order to accomplish sufficient purification of exhaust gases. For this purpose, an oxygen sensor is used to determine the oxygen concentration in exhaust gases, thereby regulating an amount of fuel to be injected into a vehicle engine in response to the determined oxygen concentration.
It is known that conventional oxygen sensors are composed generally of a container of a solid oxygen ion conductive electrolyte, such as zirconia, the inner and outer surfaces of which are provided with a metallic electrode composed of platinum or the like. The oxygen sensors of this type may measure the oxygen concentration in exhaust gases by measuring the electromotive force derived from the difference between the reference oxygen concentration produced in the container comprised of the solid electrolyte and the oxygen concentration in the exhaust gases which is brought into contact with the electrode provided on the outer surface of the container.
It is also known that such oxygen sensors are used in combination with an electronic fuel injection (EFI) apparatus or a secondary air supply apparatus, whereby the oxygen concentration in the exhaust gases in virtually balanced state can be measured. Where the oxygen concentration in balanced exhaust gas is controlled using air as the reference gas, an electromotive force of about 1 volt is produced on the exhaust gas rich side and an electromotive force of about 1/10 volt on the exhaust gas lean side. The excessive air ratio (.lambda.) in exhaust gases can be defined by the following formula: ##EQU1##
Where the excessive air ratio (.lambda.) is one, it means that air is reacted in approximately stoichiometric amounts with combustible substances in exhaust gases. The .lambda. value being more than one (.lambda.&gt;1) means that the combustible substances are lean, and the .lambda. value being less than one (.lambda.&lt;1) means that the combustible substances are rich. Where the .lambda. value is one in balanced exhaust gases, the electromotive force on an oxygen sensor rapidly changes from about 1 volt to about 1/10 volt so that even conventional oxygen sensors can detect whether air is supplied in proper amounts, thereby performing the regulation of the air excessive ratio.
As is well known, however, the composition of the exhaust gases from vehicle engines may deviate from a balanced state to a remarkably great extent. Unbalanced exhaust gases may be produced where an engine is operated while the .lambda. value is regulated below one in the air intake system and then a secondary air is injected into the exhaust system to regulate the exhaust gases at .lambda.=1 whereby to react with combustible substances present in the exhaust gases. In this case, the electromotive force of the oxygen sensor does not change as long as the excessive air ratio (.lambda.) does not exceed one and is not changed to the exhaust gas lean side. In other words, an oxygen sensor may cause to change its electromotive force where exhaust gases contain air and fuel in approximately stoichiometric amounts, that is, the .lambda. value is one, but the electromotive force of an oxygen sensor can change in unbalanced exhaust gases only when the excessive air ratio (.lambda.) is caused to be brought to the exhaust gas lean side. Therefore, in exhaust gases in unbalanced state, conventional oxygen sensors may not work nor measure the oxygen concentration therein with accuracy. Thus, conventional oxygen sensors are insufficient for use in unbalanced exhaust gases so that a computerized signal compensation circuit and expensive accessory circuits would be needed for the application of such conventional oxygen sensors to said exhaust gas disposal systems to provide satisfactory sensor performance.
Accordingly, many attempts have been made so far to improve characteristics of oxygen sensors, but the above-mentioned problems encountered with in the use of the conventional oxygen sensors in unbalanced exhaust gases remain unsolved.
Japanese Patent Application Laid-Open No. 137,591/1975 discloses an oxygen sensor in which a pelletized catalyst is provided on the outer surface of a conventional oxygen sensor to oxidize components of unbalanced exhaust gases. This oxygen sensor, however, is practically insufficient because the pelletized catalyst conventionally used for purification of exhaust gas are merely applied to the oxygen sensor. This oxygen sensor has not actually been applied to vehicles because the pelletized catalyst supported on an oxygen sensor element could not function effectively nor maintain its activity for a long period of time under circumstances under which the oxygen sensor is exposed to an exhaust gas system and it consecutively undergoes vibration of a vehicle, high temperature of the exhaust gases and heat impact or shocks. Further, the pelletized catalyst to be used therefor may tend to decrease its catalytic activity during operation due to the modification of the carrier from gamma-alumina to alpha-alumina. Moreover, the pelletized catalyst, when applied to an oxygen sensor element, may be subject to removal from the surface of the element owing to the continued vibration of a vehicle engine and the heat impact or shocks of the exhaust gases by the exhaust gases, and it is difficult to support the pelletized catalyst layer on the sensor element tightly enough to remain fit against the vehicle vibration and the heat shocks. Accordingly, the use of the PG,6 pelletized catalyst for a sensor element leaves such problems as they are and is not practical. It is further known that there is employed for this purpose an oxygen sensor in which a refractory layer (MgO.Al.sub.2 O.sub.3) is provided on the outer surface of the outer electrode of the oxygen sensor element and a platinum catalyst layer is formed on the refractory layer with another layer of a refractory material formed on the outer surface of the catalyst layer. This type of the oxygen sensor element is expensive because platinum is used in plates and the catalytic efficiency of this sensor is low because the catalyst layer is placed between the two refractory layers.
It is also known that an oxygen sensor possesses a spinel catalyst layer formed on the outer surface of the outer electrode of an oxygen sensor element. This type of the oxygen sensor element cannot achieve sufficient catalytic performance because of a low availability of catalyst.